Phase modulation



June 1, 1943.

L. R. WRATHALL PHASE MODULATIO'N Filed Nov. 4, 1941 /NVENmP' By .RWRATHALL ATTORNEY Patented June 1,. 1943 Price rrrssEMopULA'rloN Leishman R. Wratliall, River Edge, N. J assigner to Bell Telephone Laboratories,` Incorporated, New York, Y., a ccrlrioration of New York Application November 4, 1941, Serial No. 417,811

` s claims. (ci. 17e-171.5)

. This invention relates to frequency or phase modulation systems and more particularly to systems for the production of modulated waves having Wide frequencydeviations An object' of-the invention is to facilitate the production of wide frequency swings of a modulated carrierwave while at thesame time maintaining a high degree of stability of the mean frequency. linearity of the modulation characteristic in wide swing modulation systems. Additional objects are to facilitate the production of wide frequency c and thereby permits the use of frequencies with the'consequent simplication' `of the later frequency multiplication steps.

Another object is -to improve the deviations at relatively low carrier frequencies and to simplify the equipment necessary for the c production oflarge frequency deviations.

The modulation method of the invention is in some respects similar to the well-known method according to which a low frequency carrier derived from a stable frequency source is modulated in phase by the signaling current and is then increased in frequency by steps of frequency multiplication sufficiently to bring the phase swing or the frequency deviation to the desired amplitude. vention, however, a large part of the multipli# cation of the phase deviation-is made to take place in the modulator itself thereby reducing the amountof frequency multiplication subsequent to the modulation process. It has in common with theearlier systems the advantage that the stability of theA mean frequency of the transmitted Wave is thesame as that of the original carrier sourceand is not affected bythe modulation process.

The extent of the modulation imparted to the low frequency carrier in systems of the type described above is ordinarily restricted to a phase deviation of less than 30 degrees in order that a high degree of linearity of the modulation characteristic may be maintained. To increase this limited modulation to an extent sufi'lcient to provide a frequency deviation of l'I5 kilocycles 0r more requires multiplication by a factor which may -be as high as 2,000 and this may entail the use of as many as ten or even more frequency doublingv stages following the modulator. To provide; a Wide frequency deviation at'an assigned carrierrwave length may therefore require' lthe useof an initial carrier of very low frequency, in which case the multiplication processmay give rise to difficulties because of the close spacing of the harmonics of the original carrier wave. rI'hemethod of the invention provides large multiplications of the phase modulation within the modulator itself In accordance with the in-- The nature of the invention and its mode of operation will be understood from the detailed description which follows and by reference to the accompanying drawing of which:

' Fig. 1 illustrates the embodiment of the invention in a frequency modulation ratio transmitter;

andV

Fig. '2 is a diagram explanatory of the operation ofthe system.

Referring to Fig. 1, l0 is a carrier wave gener-l ator which may, for example, be a quartz crystalcontrolled vacuum tube oscillator or which may be any other source' having an equally high degree of frequency stability. The generator supplies current to a circuit comprising condenser II, inductance I2, and an impulse generating coil I3. This impulse coil may comprise a very small closed magnetic core made of fine laminations of a nickel-iron alloy containing 78.5 per cent nickel, known as Permalloy, and woundv with a few turns of wire. The speech current circuit comprising a microphone it, transformer I5, predistorting .network i6, Il and. adjustable resistance I8 is coupled to the impulse coil through a. high frequency chokev i9 in such a.v

way that the speech'currents are superimposed upon the carrier currents in the -coil windings.

thence to the input circuits of' a modulatori. v

The band-pass filters are designed to transmit different harmonics of the carrier frequency; for example, the filter 2li may be tuned to the twentieth harmonicV and .the lter 25 to the fteenth harmonic. The oscillations selected by the two filters are impressed uponmodulator 26 and from the output of this the modulation products correspondingqto the difference ofthe two frequencies are selected by band-pass filter 2l. The

remaining parts of the system comprise a mul-evv tiplier 28 in which the frequency of the carrier is raised t0 the value desired for radiation, a

band-pass filter 29, power amplifier 30 yand an antenna 3l. The multiplier 23 may contain as many individual stages as are necessary, together with suitable filters for selecting the desired frequency in the output of eachy stage. The final higher mitin filter 29 selects the harmonic that is to be utilized.

The output circuit of the carrier generatorl is tuned by means of condenser Hand inductance I2 to the frequency of the generated wave and these elements constitutesubstantially the whole impedance of the circuit. The inductance of coil I3,.as will be explained later, is of very low value during the greater part of the carrier wave cycle, so that it has substantially no eect on the tuning of the circuit. If need be, its inductance may be taken into account by an appropriate adjustment of the inductance coil l2. By virtue of the tuning oi' the circuit-the carrier current traversing the windings of coll I3 is kept substantially free from harmonics and may be regarded as having a simple sinusoidal wave form. In the operation of the device it is desirable that the amplitude oi the carrier wave should be maintained substantially greater than that of the signal wave. Resistance i 8 in the speech circuit is provided for the purpose of adjusting the amplitudes of the two currents to their proper relative values.-

The predistcrting network IS, l1 is such as to attenuate the several components of the speechV current in degrees proportional to their respective frequencies, in accordance with the well` known principle that such predistortion of the signal wave before phase modulation gives the iinal modulated wave the characteristics of frequency modulation. The rectiers 22 Yand 23 are indicated conventionally and may be diode or triode vacuum tubes, or copper-oxide rectiers. l

The operation of the system will now be explained. When the windings ofa magnetic coil are traversed by a sinusoidal alternating current of sumcient amplitude to produce strong saturation of the magnetic core, the voltage across the coil terminals develops a sharrply peaked wave form with the alternate positive and negative peaks centered close to the instants that the current passes through zero value. By the use of core materials which are easily saturated magnetically and which are relatively free from hysteresis, and by increasing the amplitude of the impressed current to a value, many times that necessary for saturation, the voltage wave can be made to take the form of a train of very short and very sharp impulses alternately positive and negative. These impulses will be practically coincident with the instants of zero current and can be made to last substantially less than one per cent of the period of the current. 4

If the impressed current consist of a sinusoidal frequency saturating current upo'n which is superimposed a relatively weak low frequency signaling current, the instants of zero value or the resulting complex current will be displaced in time from the zeroes of the high frequency component taken by itself and the timing of the impulses will be correspondingly altered. The manner in which the instants of zero current and the timing ci' the voltage impulses are affected is shown clearly in Fig. 2. Here the horizontal line X-X' representsthe time axis, sinusoidal curve A represents the instantaneous values of the high frequency saturating current, and curve B represents a portion of a cycle oi the low frequency signaling current plotted with its phase reversed. The superposition of the two components is equivalent to the substitution of the curve B as a new time axis, the zero values of the complex current occurring at the times corresponding tothe intersections of the two curves. The polarity of an impulse depends upon the sign of the time derivative of the current at the instant of zero value. In

the gure, the vertical solid lines C represent positive voltage impulses occurring as the current changes from positive to negative and the dotted lines B represent negative impulses induced as the current reverses in the opposite direction. It is evident from an inspection of the diagram that' the negative impulses are retarded in time with respect to the zero instants of the high frequency current alone, while the positive impulses are symmetrn .ally advanced in time.

The above action takes place in the coil i3 when the windings are traversed by the superimposed currents from sources i0 and l0 and the voltage impulses are developed across the coil'terminals. To develop the shortest and sharpest possible impulses -it is desirable that the core material of the coil should be such that it is very readily saturated by a weak magnetic force and also should -be substantially free from hysteresis. Materials such as the 78.5 'per cent nickel-iron alloy already mentioned are-l well adapted to meet these requirements and permit very large impulse voltages to be developed in a coil having only a lfew turns of wire inthe windings. For operation at high frequencies, of the order of 1,000,000 cycles or more,l

it is desirable that the total 'energy loss in the coils be kept as small as possible and to this end the core-should be of small volume and very finely laminated. The carrier current should be so adjusted that the magnetic core becomes saturated at a very small fraction of its amplitude. In that case the induced electromotive force in the coil is substantially zero except at the instants of current reversal and the eEective inductance of the coil over substantially the whole high frequency cycle is negligibly small.

During the existence of the induced voltage pulse, condenser 2nbecomes charged and immediately upon the termination of the pulse it discharges at a very rapid rate, the discharge current producing an impulse voltage drop across the resistance 2|. The eiect of the condenser and resistance is to provide a substantial sharpening oi the pulse and to make its duration substantially independent of the other portions of the circuit. The rate of discharge of the condenser depends principally upon the value of the resistance but may be modified to a slight extent by the inductance of the impulse coil during its saturated interval. It is not necessary for purposes of the invention that the condenser and resistance be used, and in many cases it may be preferable to employ directly the impulses representing the induced voltage across the coil terminals. For this purpose the condenser and resistance may be entirely omitted or, if desired, the condenser may be made of such large capacity that its eect is negligible.

The two trains of impulses, positive and negative, are modulated in opposite senses with respect to the times of their occurrence. It may be shown that each train is equivalent to a multiplicity of harmonically related carrier-waves, all modulated in phase by the superimposed low frequency current i`n degrees proportional to their respective frequencies. quency of the train is the same as that of the carrier current from source It and the amplitude, 0, of its phase modulation has substantially the value i =sin-ls/c 1) where S and C denote the amplitudes of the signal and carrier waves, respectively. As already The fundamental irepointed out, this value should not be permitted to exceed about 30 degrees in order that linearity harmonic component is substantially equal to the above value multiplied by the number denoting the order of the harmonic.

The rectifiers 22 and 23 provide for the separation of the two trains of modulated impulses. For example, the positive impulses may be selected by rectifier 22 and passed toband-pass filter 2B and .the negative impulses may be selected by rectier 23 and passed on to lter 25. The harmonic components selected by filters 24 and 25 will therefore be phase modulated in opposite senses and the frequency deviations corresponding to the phase modulations will likewise take place simultaneously in opposite senses. Since the selected waves are modulated in opposite senses the modulation of the difference frequency component will exhibit an increased modulation equal to the sum of the modulations of the two waves. The net result is the production of a wave in which the phase and frequency modulation is greatly multiplied without a corresponding increase in its meanfrequency. If the mth and nth harmonics be selected by the filters, vit may be shown that the l mean frequency of the waveselected bybandpass lter 2 has the value (1n-TL) c and its phase deviation has the amplitude (m-l-n) 6 where fc denotes the frequency of the original carrier current and 0 is the phase modulation of the fundamental component of the impulse trains. The fact that the two quantities m and n can be independently chosen, results `in a great degree of flexibility inthe system with respectto its capability for producing any desired degree of modulation at any assigned frequency. The choice of the initial value of the carrier wave is therefore not restricted by the amount of multiplication that must be applied to the original small phase modulation and, accordingly, relatively high values for this frequency may be chosen. For example, if we 'assume that it is desired to provide a modulation swing of about '15 kilocycles at a final carrier frequency of 40 megacycles it will be found that a multiplication of the phase modulation of about 1,000 would be required. This would normally entail the use of an original carrier having a frequency of about 40 kilocyoles persecond. By the use of the invention, however, an original carrier of 1,000,000 cycles might be chosen and the fifteenth and fourteenth harmonics of the resulting impulse trains selected. The selected output of modulator 26 will then be a wave of 1,000,000 cycles'with twenty-nine times the modulation of the fundamental. With a frequencymultiplication of V40 in the subsequent multiplier stages, the total multiplication of the modulation becomes 1160,

which is substantially the value desired. The use of an original carrier frequency 'of the order indicated ensures a great enough separation of the harmonics so that no diiculty is met in selecting any harmonic up to at least the twentieth.

In the systems of the invention the initial phase modulation of something less than degrees, which is commonly used to ensure a satisfactorily linear modulation characteristic, is` not increased, consequently the large modulation -swings are achieved without sacrifice of signal quality. On the other hand, the facility with which the large modulation swings are developed makes it possible to start with substantially reduced initial'modulation and thereby to achieve an increased degree of linearity of the final modulation characteristic.

The invention is also effective in preventing or diminishing distortion of the modulation characteristic due to hysteresis and eddy currents in the impulse coil cores. The presence of hysteresis in the core material results in a time lag between the induced voltage impulses and the instants of Zero current in the coil windings. v

Since the positive and the negative impulses are modulated in opposite senses by the signal, the

time lag, which is equivalent to a more or less constant phase bias, affects the modulations of the two impulse trains oppostely. Its effect in each train is to introduce a distortion of the modulation. characteristic corresponding to the introduction of a second harmonic of the signal current and if one of the trains, positive or negative, is used alone for the production of the'final modulated wave, the distortion vmay become noticeable. By utilizinglthe two trains in the manner described for the production of the final modulated wave, their respective distortions tend to neutralize each other thus preserving the 1inearity of the modulation characteristic."- The effect of eddy currents in the core material is like that of hysteresis, the presence of eddy currents', tending to widenthe hysteresis loop and to produce the same kind of distortion of the modulation characteristic. It is compensated in the system of the invention in thesame way as is the hysteresis eect.

What is claimed is:

1. The method of phase modulation which comprises superimposing a signal oscillation on a sinusoidal carrier oscillation of relatively g'eater amplitude, deriving from the resulting complex oscillation a train of impulses, alternately positive and negative, occurring at the instants of zero value of the saidcomplex oscillation, separating the positive and the negative impulses, selecting from the separated impulses two oscillations of different frequencies corresponding to different harmonics vof lthe carrier oscillation.

modulating the amplitude of one of said selected oscillations by the other selected oscillation, andV selecting from the resulting amplitude modulated wave the component oscillation of frequency corresponding to the frequency difference of the said selected harmonic oscillations.

2.. The method of modulation which comprises superimposing a signal oscillation on a sinusoidal carrier oscillation of relatively greater amplitude,

deriving from the resulting complex oscillation a .duce an oscillation ofthe differential frequency and. selecting the differential frequency oscillation so produced.

3. In a phase modulating system-comprising a magnetic impulse coil, a source of high frequency currents, a source of signal current, and

circuit means for superimposing the currents from said sources in the windings of said impulse coil to induce therein alternate positive and negative voltage impulses, the method which comprises separating the positive and the negative impulses, selecting a harmonic component of the separated positive impulses, selecting a different harmonic component of the negative impulses, `heating the selected components together to produce an oscillation of the differential frequency, and selecting and transmitting said differential frequency oscillation.

4. A phase modulating system comprising a source of carrier currents, a source of signal currents, a magnetic impulse coil, circuit means for superimposing the currents from said sources in the windings of said coil, means for separating according to their polarityvoltage impulses generated in said coil by the superimposed currents, selective circuits for selecting from the separated impulses two oscillations of dierent frequencies corresponding to dinerent harmonics of the carrier current, a modulating device, circuits for impressing said selecd harmonics thereon, and

a filter in the output circuit of said device for selecting oscillations corresponding to the differential beat between the impressed harmonic oscillations.

5. A phase modulating system comprising a l magnetic impulse coil, a source of carrier currents, a source of signal currents, circuit means for super-imposing the currents from said sources in the windings of said coil, a pair of transmission paths coupled to said coil to receive and transmit impulses generated therein, a rectifier in each of said paths, said rectiers being respectively poled to transmit impulses o1 opposite signs, a selective circuit in one of said paths tuned to select one harmonic of the carrier current, the selective circuit in the other of said paths tuned 4to selectV a diierent harmonic oi the carrier curcomprises deriving from a single carrier wave two trains of impulses recurring at the carrier frequency rate and modulated in their timing in opposite senses in accordance with a common signal oscillation, selecting-single harmonic-components of different frequencies from the two trains, beating the selected harmonics together to produce an oscillation of the diiierence frequency and selecting the difference frequency i oscillation so produced.

LEISHMAN R. WRATHALL 

